1. Field of the Invention
The present invention relates to a capillary used in a wire bonding apparatus.
2. Prior Art
A prior art wire bonding method will be described first.
Various types of wire bonding methods have been proposed; and the most common method is illustrated in FIG. 4.
First, as shown in FIG. 4(a), a ball 3a is formed by means of a spark discharge created by an electric torch 5 on a wire 3 which extends from the lower end of a capillary 4, and the electric torch 5 is moved in the direction indicated by the arrow. Next, as shown in FIG. 4(b), the capillary is moved to a point above the first bonding point 1a; and as shown in FIG. 4(c), the capillary 4 is lowered so that the ball 3a on the tip end of the wire 3 is pressed against the first bonding point 1a, and an ultrasonic vibration is applied to the capillary 4 by means of a horn which is provided with the capillary 4, thus bonding the ball 3a to the first bonding point.
Afterward, as shown in FIG. 4(d), the capillary 4 is raised. Then, as shown in FIG. 4(e), the capillary 4 is moved in the looping direction A to a point above the second bonding point 2a. Then, as shown in FIG. 4(f), the capillary 4 is lowered so that the wire 3 is pressed against the second bonding point 2a, and an ultrasonic vibration is applied to the capillary 4 by the horn, thus bonding the wire 3 to the second bonding point. After the capillary 4 is raised to a given position, a damper 6 is closed, and the capillary 4 and damper 6 are raised together so that the wire 3 is cut as shown in FIG. 4(g). As a result of this operation, the connection of one wire is completed.
Japanese Patent Application Kokai (Laid-Open) No. S57-87143 and Japanese Patent Application Kokoku (Publication) No. H1-2653 1, for instance, disclose wire bonding methods of this type.
One type of conventional capillaries has, as shown in FIG. 2, a chamfer 11 formed in the tip end of the wire threading hole 10, and another type of conventional capillaries has, as shown in FIG. 3, two chamfers, i.e., lower and upper chamfers 11 and 12, which are formed in the tip end of the wire threading hole 10. In these Figures, HD indicates the hole diameter, T indicates the tip diameter, CD indicates the chamfer diameter of the lower chamfer 11, .theta..sub.1 indicates the chamfer angle of the lower chamfer 11; .theta..sub.2 indicates the chamfer angle of the upper chamfer 12, .alpha. indicates the face angle, and OR indicates the outer radius. Where d is the diameter of the wire 3, then the hole diameter HD is d+(8 to 12) .mu.m (microns); in addition, .theta..sub.1 is 80 to 100 degrees, and .theta..sub.2 is 20 to 40 degrees.
Examples of capillaries of the type described above are disclosed in Japanese Utility Model Application Kokoku (Laid-Open) No. HI-42349 and Japanese Patent Application Kokoku (Publication) No. H3-780, etc.
The prior art capillary 4 described above which includes two chamfers, i.e., lower and upper chamfers 11 and 12, has some problems. When the ball 3a is bonded to the first bonding point 1a shown in FIG. 4, since the capillary 4 has the upper chamfer 12 as shown in FIG. 3, the total volume inside the chamfers 11 and 12 is great, and a high amount of material of the ball 3a enters the area of the upper chamfer 12. As a result, the amount of ball material outside the upper chamfer 12 becomes small, so that the diameter of the press-bonded ball becomes inevitably small.
Recently, the pads on pellets are installed at finer pitches due to a strong demand of a higher integration of semiconductor IC chips; and therefore, the diameter of the press-bonded ball becomes smaller relative to the diameter of the wire 3. In the prior art described above, the hole diameter HD of the capillary is set to be 8 to 12 microns greater than the wire diameter d, and two chamfers, i.e., lower and upper chamfers 11 and 12, are formed so that the volume of the upper chamfer 12 is greater than the lower chamfer 11; as result, it is possible to make the diameter of the press-bonded ball small when the ball 3a is bonded to the first bonding point 1a shown in FIG. 4.
The prior art described above can be used for a wire that has the diameter d in the range of 20 to 80 microns; and in cases where the diameter d of the wire 3 is as small as 10 to 30 microns, various problems arise.
In particular, when the hole diameter HD is 8 to 12 microns greater than the diameter d of the wire 3, and the chamfer angle .theta..sub.2 of the upper chamfer 12 is as large as 20 to 40 degrees, then the ball 3a must inevitably be formed small in size as described above in order to reduce the diameter of the press-bonded ball in accordance with the demand of fine pitches in the semiconductor IC chips. However, when the hole diameter HD is 8 to 12 microns greater than the diameter d of the wire 3, and the chamfer angle .theta..sub.2 of the upper chamfer 12 is as large as 20 to 40 degrees as described above, the ball is contacted by the edge part 13 which is at the lower end of the chamfer 12.
When bonding is performed under such conditions, the ball 3a is cut by the edge part 13 in the initial stage when the capillary 4 is lowered; and the portion of the ball directly beneath the upper chamfer 12 enters the wire threading hole 10 through the upper chamfer 12, while the remaining portions of the ball gradually make contact with the lower chamfer 11 and then finally contact the entire surface of the lower chamfer 11; afterward, the ball material enters the area defined by the face angle .alpha.. In this case, the force required for bonding cannot be transmitted to the portions of the ball which have entered the area of the chamfer 12 and the interior of the wire threading hole 10. As a result, the force required for bonding can be transmitted only to the portions of the ball located inside the area of the lower chamfer 11 and in the area beneath the face angle .alpha.; and the bonding force drops if the amount of ball material that has entered the area of the chamfer 12 and the interior of the wire threading hole 10 is large. Furthermore, since the portions of the ball material that have entered the area of the chamfer 12 and the wire threading hole 10 contact the side surfaces of the wire threading hole 10 in the vertical direction, friction is generated when the capillary 4 is raised. This produces a force which peels off the press-bonded ball bonded to the first bonding point la, and thus weakens the press-bonding force.